Integrated optical devices for directly processing optical signals have become increasingly important as optical fiber communications increasingly replace metal cable and microwave transmission links. Integrated optical devices can be implemented as silica optical circuits which employ integrated glass waveguide structures formed on silicon substrates. The basic structure of such devices is described in C. H. Henry et al., "Glass Waveguides on Silicon for Hybrid Optical Packaging", 7 J. Lightwave Technol., pp. 1530-1539 (1989) (Henry et al. reference), which is herein incorporated by reference.
Typically, in silica optical circuits, a silicon substrate is provided with a base layer of SiO.sub.2, and a thin core layer of doped silica glass is deposited on the SiO.sub.2 layer. The core layer can be configured to a desired waveguide structure using standard photolithographic techniques. Then, a layer of doped silica glass is deposited over the core layer to act as a top cladding. Waveguides formed in silica optical circuits have been configured to provide optical switching.
A known thermo-optic single mode switch configuration is described in M. Kawachi, "Silica Waveguides on Silicon and Their application to integrated-optic components", 22 Optical and Quantum Electronics, pp. 391-416 (1990) (Kawachi reference), which is incorporated by reference herein. This optical switch uses a Mach-Zehnder interferometer configuration and includes first and second directional couplers and two equal-length single-mode waveguide arms buried within silica glass. Each coupler has a pair of inputs and a pair of outputs. The two waveguide arms connect the outputs of the first coupler to the respective inputs of the second coupler. Thin chromium heater strips are deposited on the surface of the top cladding above the waveguide arms.
In operation, when no electricity is applied to either of the heater strips, a light signal at one of the inputs of the first coupler is split such that a portion of the signal is transmitted through each waveguide arm to the second coupler. The second coupler recombines the light signal portions and directs the recombined light signal to a particular output of that coupler. When electricity is applied to one of the heater strips, the heater strip heats the corresponding waveguide arm which increases its index of refraction and correspondingly the effective length of the light path through that waveguide arm. The increase in the effective length of the light path produces a phase difference between the light signals at the end of that waveguide arm and the end of the unheated waveguide arm. When the phase difference is .pi. radians, destructive interference occurs in the second coupler and the routing of the recombined light signal is switched to the other output of the second coupler.
Other known types of thermo-optic switches that rely on heater strips to perform the switching function are disclosed in M. Haruna et al., "Thermoptic Deflection and Switching in Glass", Appl. Opt., Vol. 21, pp. 3461-65 (October 1994) (Haruna reference), which is incorporated by reference herein.
The packing density of these known switch configurations is limited due to the area required for the heater strip contact electrodes. The heater contacts provide a soldering or bonding pad for connection to electrical wires from heater drive circuits. Moreover, such switches cannot be positioned too close to one another because the heat generated by a particular heater strip can undesirably effect the index of refraction of a waveguide of an adjacently positioned switch. As a consequence, a multiple port (M.times.N) switch configuration having a large number of individual or cascaded conventional switches requires a relatively large portion of an integrated circuit chip and has a relatively high wiring complexity. In addition, each of the corresponding chromium heater strips requires power on the order of 0.5 W to switch light paths which can result in a high power requirement for cascaded multiple port switches.
Thus, there is a recognized need for relatively low cost silica optical circuit switch that can be implemented in compact dimensions without the need for strip heaters or heater electrodes.